A. Field of the Invention
The present invention relates to an improved process for producing carbon fibers (hereinafter including graphite fibers also) which is excellent in operational stability, and more particularly to a process for producing carbon fibers which comprises thermally stabilizing and carbonizing acrylic fibers (including precursor fibers in filament form or in tow form) containing a predetermined amount of a straight chain silicone substance and a specific chemical substance, whereby high quality carbon fibers (carbon fiber filaments or tows) can be obtained and the operational stability in the step of heat treatment can be heightened.
B. Discussion of the Prior Art
It is already known that carbon fibers can be obtained by thermally stabilizing acrylic fibers in an oxidizing atmosphere at 200.degree.-300.degree. C., and then carbonizing the thus thermally stabilized fibers in a non-oxidizing atmosphere. However, what should be noted here is that the thermal stabilization reaction (oxidizing reaction) of acrylic fibers is an exothermal reaction, so that if the fibers are heated rapidly, local accumulation of heat takes place which is liable to cause an uneven reaction. Consequently, the fibers will fuse together or become brittle in the thermal stabilization step, and it is difficult to obtain high quality carbon fibers. Of course, various attempts have been made to remedy such technical defects. Such attempts include, for example, a method wherein the thermal stabilization is carried out at low temperatures for a long time, and a method wherein precursor fibers are impregnated with or caused to contain an organic silicone substance and then thermally stabilized, as described in Japanese Patent Laid-Open (Kokai) Application No. 117724/1974. However, in fact, these methods still involve unsolved problems. Namely, when the particular silicone substance as mentioned above is employed, the fusion or agglutination of acrylic fibers can be reduced to some extent, but on the other hand, owing to the water repellency of the silicone substance used, the acrylic fibers given such a substance tends to generate static electricity. When static electricity is generated, serious troubles such as fiber entanglement upon drawing out the fibers, winding of fibers around rollers or guides in the steps of thermal stabilization and carbonization, generation of fluff, etc. are caused and make the operation extremely unstable. To avoid such troubles, attempts were made to give the fibers antistatic spinning oils (anionic surface-active agents such as salts of higher fatty acids, higher alkyl sulfates, etc; cationic surface-active agents such as higher alkyl amine salts, etc.; nonionic surface-active agents such as condensation products of higher alkyl fatty acids with allyl alcohol or glycol) but when a usual spinning oil is used, it turns into a tar-like substance in the course of the thermal stabilization step, and a large amount of the heat-decomposed substance remains on the surface of the fibers. Therefore, the phenomenon of fiber fusion or agglutination occurs again and disadvantages such as fiber breakage, etc. are caused. Among others, when an acrylic fiber tow produced by wet-spinning is used as the starting material, the form of the tow is not only remarkably disordered by the repulsive power among single fibers owing to static electricity, but also aggulutination or fusion by the tar-like substance is frequently caused, and it has been difficult to obtain satisfactory carbon fibers.